In the process of manufacturing a plate, such as a steel plate, the plate is subjected to rolling and cooling steps, in which the plate undergoes deformation, such as warping and/or waving. Accordingly, in order to remedy the deformation, such as warping and/or waving, and thereby to flatten the plate, a roller leveler is used, which includes a plurality of leveling rolls disposed on upper and lower sides in a staggered manner.
The roller leveler passes a plate to be flattened, with the upper leveling rolls being caused to penetrate between the lower leveling rolls or the lower leveling rolls being caused to penetrate between the upper leveling rolls, to repeatedly apply bending to the plate, and thereby to planarize the warping and/or waving of the plate. In general, a plurality of lower leveling rolls and a plurality of upper leveling rolls are supported by respective roll frames and flattening of a plate is performed by pushing the upper leveling rolls via pressing cylinders (hereinafter also referred to as pushing cylinders) provided both in the entrance side and the discharge side, with the lower leveling rolls fixed.
In the process of flattening a plate, the leveling rolls are driven by driving motors and, upon contact between the leveling rolls and the plate to be flattened, driving force is transmitted to the plate, which is caught between the upper and lower leveling rolls. When this is performed, the amount of penetration, or the penetration amount (hereinafter also referred to as the amount of pressing, or the pressing amount), of the upper leveling rolls by pressing cylinders is set according to various conditions, such as the thickness, material, and shape of the plate, and the diameter and roll pitch of the leveling rolls, so that required flatness is obtained.
In the meantime, the plates to be flattened, which are metal plates, such as steel plates, generally include a plate with wavy deformation, that is, edge waves, at edge portions with respect to the plate width direction. The edge waves occur due to the following three causes:
(1) Unevenness in roll gaps in a rolling step (edge portions are relatively strongly rolled);
(2) Unevenness in cooling after hot rolling; and
(3) Rolling or flattening of the material, in which yield stress in edge portions with respect to the plate width direction is lower than a center portion with respect to the plate width direction.
It is considered as a problem that, when a plate with a thickness of 6 to 10 mm, in which there are edge waves, is subjected to a flattening process using a roller leveler having large-diameter rolls with a diameter of 360 mm or so, the plate is not flattened or the edge waves therein increase. Such a leveler has been used in many cases in recent years. Specifically, since the yield stress in edge portions with respect to the plate width direction is lower than that in a center portion with respect to the plate width direction, the amount of elongation is greater at the edge portions of the plate with respect to the plate width direction. For this reason, even when there is no edge wave before flattening the plate, edge waves can occur during the flattening process. If there were already the edge waves, the degree of unevenness of the edge waves would further increase. Consequently, when a roller leveler that has large-diameter rolls with a diameter of 360 mm or so, it is difficult to flatten the plate with a thickness of 6 to 10 mm, in which there are the edge waves, or the plate with a thickness of 6 to 10 mm, in which the range of variation in yield stress is greater than 50 MPa or so in the plate width direction even though there is no edge wave.
A method of flattening a plate, in which the plate is flattened while the leveling rolls are bent in the longitudinal direction, is proposed as a technology for flattening a plate with a thickness of 6 to 10 mm, in which there are edge waves that are wavy deformation in edge portions of the plate with respect to the plate width direction (see Patent Document 1 or 2, for example).